This invention pertains to nonreciprocal optical devices such as optical isolators and optical circulators constructed from materials that exhibit the optical Faraday effect. Optical isolators are commonly used to overcome the instability in semiconductor light sources caused by reflected light. Optical circulators may be used in two way optical fiber communication systems and in other applications. In particular, the invention pertains to quasi-achromatic optical isolators and circulators.
Nonreciprocal optical devices such as isolators and circulators may be constructed from materials that exhibit the optical Faraday effect. This effect is a circular birefringence that arises from the presence within the material of a magnetization resulting from an externally applied magnetic field or from an internal spontaneous magnetization due to ferromagnetic or ferrimagnetic ordering that may be held in a saturated state by an externally applied magnetic field. In either case it manifests itself as an optical rotatory effect upon light propagating through the material along the direction of magnetization. It is nonreciprocal in that the sense of rotation of the axes of polarization depends on the polarity of the magnetization relative to the direction of propagation.
Optical signals transmitted through fiber optic waveguides are being used for telecommunications to an ever increasing extent. They are generated by laser diodes of various types that often operate at wavelengths in the 1.28 to 1.60 .mu.m range. Some of these lasers, especially those of the so-called distributed feedback construction are somewhat sensitive to light returning on their output fiber, whether it be from reflections of their own emissions or from another source. An optical isolator which is a nonreciprocal two-port device that passes light in one direction and absorbs light in the opposite direction, is often necessary to obtain optimum operation from these laser diode sources.
The optical circulator is a more generally applicable nonreciprocal four-port device. As with the isolator light entering the first port passes out the second port, but light entering the second port is not absorbed, and instead passes out the third port. Similarly, light entering the third port passes out the fourth port, and light entering the fourth port passes out the first port. Thus by using any two adjacent ports, a circulator can function as an isolator, but it also has the potential of permitting optical fiber transmission lines to be operated in a bidirectional mode with signals at the same wavelength traveling in opposite directions simultaneously.
Basic to the operation of both optical isolators and circulators is the 45 degree Faraday rotation element which is usually composed of glass or a single crystal transparent over the desired wavelength range. Opposing parallel optical facets surround the active region which is within an externally applied axial magnetic field provided by adjacent permanent magnets or by a current carrying solenoid. The field strength required to obtain 45 degrees of rotation depends on the Verdet constant of the element material. Suitable materials include diamagnetic glasses especially those with a high lead oxide content, paramagnetic glasses or cubic crystals containing ions such as trivalent cerium or terbium, and ferrimagnetic oxide crystals such as yttrium iron garnet. The latter, commonly known as YIG, is especially useful in the 1.28 to 1.60 .mu.m wavelength range where many optical fiber systems operate.
In its simplest form an optical isolator consists of an input plane polarizer, a 45 degree Faraday element with its associated axial field magnet and an output plane polarizer with its polarization axis rotationally orientated at 45 degrees relative to that of the input polarizer. A compact isolator of this type using a YIG crystal has been described in the prior art. Input light must be plane polarized to pass through the input polarizer after which its plane of polarization is rotated 45 degrees by the Faraday element so that it can pass through the output polarizer. If the propagation direction is reversed the Faraday element will rotate -45 degrees and the light passed through it will be absorbed in the output polarizer. A similar optical circulator, also using a YIG crystal, but with input and output polarizing beam splitters instead of plane polarizers has also been described. But both devices require specific states of plane polarization at their ports to function optimally.
An isolator used immediately adjacent to a laser diode transmitter can accept its plane polarized output, but if isolators or circulators are to be generally applicable in optical fiber systems they must function with any polarization state at their ports. Polarization independent circulators have been built which use the same polarizing beam splitters to separate the two orthogonal components of any arbitrary input state so that they may be processed in parallel in the Faraday nonreciprocal element. A simpler birefringent wedge polarization splitter has been used to construct a polarization insensitive isolator, but it does not appear to be applicable to circulators.
The degree of isolation obtainable with either of these nonreciprocal devices is limited by deviations of the Faraday element rotation from its nominal 45 degrees. The element is designed for some nominal wavelength and in general it will have a greater rotation at shorter and a lesser rotation at longer wavelengths. Also, some Faraday elements such as YIG are temperature sensitive so the rotation will change due to temperature variations. Various techniques have been used to improve the degree of isolation by minimizing these deviations from 45 degree rotation In the case of YIG, gadolinium substitution for part of the yttrium lowers the temperature coefficient of the rotation, but at the expense of its magnitude. The wavelength dependence can be partially compensated by a second element having -45 degrees of reciprocal type rotation. Such an element can be made from an optically active crystal. The two element combination between crossed polarizers would be used as an isolator. For one direction of propagation the opposite rotations would always sum to zero if they had identical wavelength dependences. But for the opposite direction of propagation both elements would have -45 degrees of rotation which would sum to -90 degrees with a doubled wavelength variation. The isolator would therefore have a wavelength dependent insertion loss. The two element combination could not be used at all in an optical circulator because isolation between all four adjacent ports could not be achieved.
The basic optical isolator of the prior art is shown in FIG. 1. An input light beam 12 propagates along the +z axis in a right-hand coordinate system and passes in turn through plane polarizer 14. Faraday rotation element 15, and output plane polarizer 18. Beam 12 is plane polarized at an angle of zero degrees to the x axis and passes through polarizer 14 unchanged. Within Faraday rotation element 15 which includes its axial field producing magnet, the plane of polarization is rotated to an angle of +45 degrees from the x axis. Output polarizer 18 is oriented at +45 degrees to pass beam 12 undiminished in intensity. A reverse direction beam would initially be polarized at +45 degrees so as to pass through polarizer 18 unchanged. Within Faraday rotation element 15 its polarization direction would be rotated to an angle of +90 degrees so that it would be completely absorbed by polarizer 14. Thus the device functions as an isolator because it transmits light propagating in the +z direction and absorbs light propagating in the -z direction.
A rotator element is considered to have a positive rotation if the polarization axes rotate in a counter-clockwise direction as the oncoming light beam is observed. In the above description of a simple optical isolator, the polarization axes were rotated from x toward y for both propagation directions, but according to the above definition this corresponds to a +45 degree rotation for the +z direction and a -45 degree rotation for the -z direction. This is the nonreciprocal behavior of the Faraday effect. A reciprocal rotation element on the other hand has the same polarity of rotation for both propagation directions. Light propagating in an optically active element for example, would have its polarization axes rotated, but if the direction were reversed the polarization axes would retrace the orientations traced during their forward path.
In the prior art optical isolator shown in FIG. 2, these two types of rotations are used together to partially compensate for the wavelength dependence cf the Faraday effect. It also requires input beam 22 to be plane polarized at an angle of zero degrees to the x axis so that it passes through plane polarizer 24 unchanged. Within Faraday rotation element 25 which includes its axial field producing magnet, the plane of polarization is rotated to an angle of +45 degrees from x axis. Within +45 degree reciprocal rotation element 27 the plane of polarization is rotated to +90 degrees from the x axis. Output polarizer 28 is oriented at +90 degrees to pass beam 22 undiminished in intensity. A reverse direction beam would initially be polarized at +90 degrees so as to pass through polarizer 28 unchanged. After passing back through the reciprocal rotation element 27 the plane of polarization would again be at +45 degrees from the x axis, but after passing back through Faraday rotation element 25, it would be rotated to an angle of +90 degrees so that it would be completely absorbed by polarizer 24. Thus this device also functions as an isolator. The purpose of the additional +45 degree reciprocal rotation element is to add the wavelength dependence of its +45 degree rotation to that of the -45 degree Faraday rotation acting upon the reverse direction beam, thereby at least partially compensating the device for operation with an improved degree of isolation over some wavelength range. But in the forward direction these wavelength dependences add and cause some wavelength dependent degree of misalignment between the plane of polarization and the orientation of output polarizer 28. This leads to a wavelength dependent insertion loss.